At least 15 also reported injured in Alexandria clashes. Deposed Egyptian president also accused of murder and kidnapping; Muslim Brotherhood dismisses accusations as 'ridiculous.'

Egyptian authorities have detained President Mohammed Morsi for 15 days over an array of accusations, including killing soldiers and conspiring with the Palestinian group Hamas, the state news agency reported on Friday.

The report came just hours before mass rallies for and against Egypt's first freely elected leader, who was ousted by the military on July 3, were expected to take place. Morsi has been held by the military since his downfall.

Thousands of anti-Morsi protesters gathered in Cairo's Tahrir Square on Friday, where army tanks and armored vehicles were deployed under pro-al-Sissi banners. Supporters of Morsi and the Brotherhood, meanwhile, congregated in the Rabaa al-Adawiya area of north-eastern Cairo. In al-Arish in Sinai, thousands rallying against Morsi carried posters of al-Sissi.

Ten people were reportedly injured in the Cairo district of Shubra on Friday, as well as six in the northern city of Damietta. Egypt's state news agency said that 15 people were injured in clashes between pro- and anti-Morsi protesters in Alexandria later on Friday. The state news agency reported that Morsi supporters threw molotov cocktails and used live ammunition.

Egypt's Muslim Brotherhood dismissed as "ridiculous" the accusations leveled by the authorities against Morsi. "They are not taken seriously at all. We are continuing our protests on the streets. In fact we believe that more people will realize what this regime really represents - a return of the old state of Mubarak, with brute force," Brotherhood spokesman Gehad El-Haddad said.

Morsi has been held by the army in an undisclosed location since he was removed from power on July 3. His supporters believe that he is being held at Republican Guard headquarters in Cairo. Some 24 hours after his last public appearance, a 30-minute televised speech on state-run television on July 2, the deposed president fell into the hands of the army and hasn't been heard from since.

At the beginning of this week, Morsi's family held a press conference where they accused the army of kidnapping him. His son said that the toppled president had not been in contact with his attorneys, his family or his friends since the army took him. World leaders have joined Morsi's family in calling for his release, saying that he should not be held unless he faces a legal indictment.

Until Friday's step by an investigating judge, Morsi had not faced any formal legal measures. The charges relate to his escape, along with other top Brotherhood leaders, from a prison north of Cairo.

The report on the state news agency said investigating judge Hassan Samir had confronted Morsi with evidence during his questioning. It did not say when or where he had been questioned.

The accusations listed against Morsi included arson, destruction of prison records and "collaboration with Hamas to undertake aggressive acts in the country, attacking police facilities, officers and soldiers."

It also accused him of "killing some prisoners, officers and soldiers deliberately and with prior intent". It added the accusation of "kidnapping some officers and soldiers".

The prosecutor has issued a gag order stipulating the media may only publish his statements on the case, citing the secrecy of the investigations and "national security."

A spokesman for the Muslim Brotherhood, which says the army has staged a coup against the democratically elected head of state, described the accusations as "ridiculous". Gehad El-Haddad said they marked the return of the "old regime".

Morsi and many other Brotherhood leaders were rounded up by the authorities during the 2011 uprising that swept Hosni Mubarak from power.

Cairo on high alert

The announcement of Morsi's arrest came amid expectations of mass demonstrations by both supporters and opponents of the revolution on Friday in Egypt and heightened fears of violent confrontations. Emergency response services were on high alert in Egypt and more than 200 ambulances were placed on-call in Cairo to take the injured to hospitals in the capital.

Egyptian army Commander-in-Chief and Defense Minister Abdul Fattah al-Sisi called upon citizens to demonstrate in support of the army and its war on terror. The Egyptian army issued a warning Thursday night to the Muslim Brotherhood with the title, “Last Chance for Supporters of President Morsi to Halt the Protest,” in which it said that the army would take harsh actions against those that advanced violence.

The Muslim Brotherhood responded with fury to al-Sisi's announcement and dubbed it a declaration of war on the Islamist movement and Morsi's political legitimacy, saying it would lead to a civil war.

The Brotherhood's spiritual leader, Mohammed Badie, who went underground after an arrest warrant was issued against him, called al-Sisi's announcement a war against Islam. He stated that it was equivalent in its severity to that of a person who destroys the Kaaba in Mecca, the holiest site in Islam. Badie instructed Morsi supporters to head to the streets to demonstrate against the president's opponents.

Observers were expecting the demonstrations Friday evening to be a test of both sides' strength, with neither side displaying any willingness to stand down.

In related news, Hamas decided on Thursday to close down the Gaza offices of the Palestinian news agency Maan and the TV network Al-Arabiya for what it called their biased coverage of the events in Egypt. A senior Maan editor told Haaretz that the decision was made due to the translation of an article that opposed the Muslim Brotherhood taken from American and Israeli press.

Lens, a Gaza-based production company, was also shut down for "collaboration with Zionist media."